1. Field of the Invention
The present invention relates to a combination of a piston and a piston ring for automotive engines.
2. Description of the Related Art
Demands for high output as well as low exhaust gas emissions and low fuel consumption in automobile engines have led to demands for piston rings with better gas sealing and oil control functions as well as low friction. Progress is underway to lower friction by making the oil ring thinner to improve conformability characteristics and also by lowering the tension. Oil rings are mainly a three-piece type combined oil ring with separated upper and lower rails; and a two-piece type combined oil ring where the upper and lower rails are connected by a web into one piece. In both of these combined oil rings, an expander presses the rail section against the inner wall of the cylinder bore, and that surface pressure acts to control the oil film by scraping off oil on the cylinder bore.
The two-piece type combined oil ring with a coil expander on the rear side has a low spring constant and also little reduced tension from wear, and the initial tension of the ring where the reduced tension is estimated can be set to a small level. Therefore, the two-piece type combined oil ring is effective for reducing friction.
However, in the conventional two-piece type combined oil ring, the oil ring usually includes two rails as well as oil holes between the rails, so that making thinner dimensions results in small oil holes, causing the problems that along with narrower oil passages, machining the holes becomes difficult. The improvement in conformability by the thinner width dimensions was therefore limited.
Technology is disclosed in Japanese patent non-examined publication No. 04-078375 for an oil ring without oil holes penetrating along the radial direction between the two rails of the oil ring; and formed with a groove extending along the radial direction to an extent not penetrating through the piston on the lower surface of the ring groove of the piston where the oil ring is mounted. Since there is no oil holes, the ring side surfaces have better precision, the ring is strong when the width is thinned, and a reverse flow of oil from the oil holes is prevented. Moreover, the groove is formed to extend along the radial direction to an extent not penetrating through the piston on the lower surface of the ring groove so that pressure is reduced on the rear side of the oil ring, and along with reducing outer circumferential wear on the oil ring, carbon sludge deposits are prevented.
Technology is disclosed in Japanese patent non-examined publication No. 11-236853 for a piston formed with a step section on the piston land section between the oil ring and the second ring, and formed with oil return holes connecting the step section and the piston internal space, and the second ring is a resin ring without a gap. Using a resin ring without a gap reduces blow-by gases, and the step section and the oil return hole return surplus oil to the crankcase so that oil consumption is reduced.
Technology is disclosed in Japanese patent non-examined publication No. 09-196172 for a piston including an oil return passage on the lower surface of the ring groove, and a combined oil ring including one rail protruding on the outer circumferential center section of the oil ring and with no oil return holes in the oil ring. This technology suppresses oil scraping effect during the upward piston stroke, and increases the oil scraping effect during the downward piston stroke, so that oil consumption is reduced.
Technology is disclosed in Japanese patent non-examined publication No. 05-280417 for a piston structure formed with a notch formed in the second land section and opening on the upper and lower grooves. The notch is formed in sections along the periphery and so sufficient space between the cylinder wall and the second land section is ensured while maintaining the oil ring position, and proper oil scrape-off performance by the oil ring is maintained, so that oil consumption is reduced.
Technology is disclosed in Japanese patent publication No. 47-020653 for a piston ring where along with closing an inner circumferential surface of a ring-shaped piece having a U-shaped cross sectional oil passage and both ends facing a gap, a connection hole is formed on the bottom plate for connecting an oil return hole formed on the piston. This structure effectively renders both an oil ring scrape off function and a gas leak prevention function.
However, when the oil ring disclosed in Japanese patent non-examined publication No. 04-078375 is made to thinner dimensions, the space between the rails becomes narrower, and sufficient space to allow oil scraped off by the upper rail to escape cannot be obtained because there is no oil holes. An increase in the oil pressure will therefore cause the problem of a reverse flow of oil upwards. The technology in Japanese patent non-examined publication No. 11-236853 utilizes a second ring with two rails and an oil ring with two rails and so is impossible to make thinner. The technologies disclosed in Japanese patent non-examined publication No. 09-196172, Japanese patent non-examined publication No. 05-280417, and Japanese patent publication No. 47-020653 do not disclose a structure for an oil path groove formed on the upper and lower surfaces of the lowermost ring groove of the piston and extending to the outer circumference of the piston, and an oil path groove formed on the lower surface of the ring groove just above the lowermost ring groove and extending to the outer circumference of the piston.